We are studying the genetic processes of mammalian cells in general and of cells involved in the immune system in particular. Our specific aims for the next five years are: 1. To obtain intra and inter specific cell hybrids by positive immunoselection. If successful, we will follow segregation and therefore gene linkage and gene interactions for different surface antigens. 2. To investigate the mode of action of mouse immune response (IR) genes, particularly the one newly discovered by us (probably to be called IR-6), which is closely linked to structural genes for immunoglobulin heavy chains. 3. To continue to investigate cell interactions in the immune response, particularly the role and mechanism of action of thymus-derived (T) cells in positively or negatively cooperating with antibody-forming-cell-precursor (B) cells. 4. To study the genetics, immune mechanisms and relationships between reticulum cell sarcomas and regulation of the immune response in the SOL/J mouse strain. For this work we will make especial use of immunoglobulin allotypes and surface alloantigens as markers, congenic mouse strains which differ genetically only for one or more of these markers and our recently developed fluorescence-activated electronic cell sorters which separate viable, functional cells along functional lines.